Pluto has proven itself to be a diverse, dynamic and fascinating world packed with strange phenomena that planetary scientists are having problems fully explaining. And now they've found an icy oddity that highlights one of the key processes that is believed to shape this dwarf planet's surprisingly young surface.

In recent high-resolution images sent back to Earth from NASA's New Horizons mission, which buzzed Pluto on July 14, 2015, a rather obvious chunk of ice appears to have been ripped away from Pluto's surface. The upper layers of methane ice are missing from a western plain called Piri Planitia, a feature that is enriched with water ice.

Below Piri Planitia is a very old cratered terrain called Vega Terra, which is separated from the young plain by a series of cliffs (or "scarps"). Some of these scarps appear to have broken off, creating isolated hills (or "mesas"). But what has caused this discontinuity? Why has the methane ice-rich surface layers been cut off by the cliffs, giving way to a lowland plain rich in water ice?

Compositional data from the New Horizons spacecraft's Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument, shown in the right inset, indicate that the plateau uplands south of Piri Rupes are rich in methane ice (shown in false color as purple). Scientists speculate that sublimation of methane may be causing the plateau material to erode along the face of the cliffs, causing them to retreat south and leave the plains of Piri Planitia in their wake. |
NASA/JHUAPL/SwRI

As we can probably guess, Pluto is a cold place. Though it's frozen, it is far from static. Chemicals that would normally be in a gas or liquid state here on Earth become solid layers of ice on Pluto. When the world receives even the tiniest amount of heating, volatiles - such as methane or ammonia ice - sublimate.

Sublimation occurs when the atmospheric pressure or temperature (or both) are very low. Water ice, for example, at sea level pressures on Earth will melt into a liquid and then boil into a vapor (or gas) when heated. On Pluto, because it's so cold and the pressures are a near vacuum, liquid water is not possible and heated water ice will sublimate directly from a solid into a gas. And it appears that's what's happening with the methane ice in Piri Planitia; it's sublimating into Pluto's thin atmosphere, likely contributing to the dwarf planet's atmospheric cycles of surface ices and exposing water ice-rich layers below.

It's almost as if Pluto is getting a rejuvenating facial scrub; over millions of years, as its surface is gently heated by our distant sun, the surface layers of ice are sublimated away, creating young-looking regions curiously free of ancient impact craters.

Newly returned pictures taken by NASA’s New Horizons spacecraft are giving scientists -- and the rest of us -- the most detailed views of Pluto’s stunningly diverse landscape. “We continue to be amazed by what we see,” NASA chief scientist John Grunsfeld said in a statement. The latest images form a strip 50 miles wide and were taken when New Horizons was about 15 minutes away from its closest approach to Pluto on July 14.

As NASA’s New Horizons spacecraft raced toward a July 14 close encounter with Pluto, the probe’s telescopic long-range camera got to work on a sequence of pictures that revealed features smaller than half of a city block. Pluto’s surface turned out to be unexpectedly diverse, evidence of a complicated and rich geology. The mosaic pictured here starts about 500 miles northwest of Pluto's smooth Sputnik Planum region and covers the rugged al-Idrisi mountains, the shoreline of Sputnik Planum and its icy plains.

This image has been scaled and rotated, for the full, high-resolution view,

Scientists aren’t sure yet how some of Pluto’s craters came to contain layers, such as the one picture here in the upper right. “Layers in geology usually mean an important change in composition or event, but at the moment New Horizons team members don’t know if they are seeing local, regional or global layering,” NASA said. New Horizons’ Long Range Reconnaissance Imager (LORRI) took a series of images about 15 minutes before the spacecraft’s July 14 close encounter with Pluto. The dark crater at the center of the image is apparently younger than the others because material thrown out by the impact is still visible. Most of the craters are within a 155-mile wide region known as Burney Basin, the outer rim of which appears as a line of hills at the bottom of this image.

New Horizons gathered a 50-mile-wide view of Pluto’s rugged northern hemisphere, including a 1.2-mile high cliff, seen here from the left to the upper right, during a series of pictures taken by the spacecraft’s telescopic Long Range Reconnaissance Imager (LORRI) on July 14. The cliff is part of a canyon system that stretches for hundreds of miles across Pluto’s northern hemisphere. Scientists believe the mountains in the middle are comprised of water ice that has been changed by the motion of nitrogen or other exotic ice glaciers over the eons. At the bottom of the image, which was taken when New Horizons was about 10,000 miles from Pluto, the badlands meet the giant icy plains of Sputnik Planum.

Blocks of ice, upper left, appeared to be jammed together in an area the New Horizon scientists are calling the al-Idrisi mountains. Some of the mountains seem to be coated with a dark material, while others are bright. Scientists think material crushed between the mountains may be from the ice blocks jostling back and forth. The mountains end at the shoreline of a region named Sputnik Planum, which is marked by soft, nitrogen-rich ices that form a nearly flat surface. New Horizon’s Long Range Reconnaissance Imager (LORRI) took a series of images in the span of about one minute at 11:36 Universal Time on July 14, about 15 minutes before the spacecraft’s closest approach.